Optogenetics is the combination of genetic and optical methods to control specific events in targeted cells of living tissue, even within freely moving mammals and other animals, with the temporal precision (millisecond-timescale) needed to keep pace with functioning intact biological systems. One application of optogenetics relates to neural stimulation. However, questions remain on the most efficient techniques to alter neural activity with photon sensitive ion channels, and quantitative tools to predict the spread of optical stimulation are lacking.